1. Field of the Invention
The present invention relates to an image forming apparatus and, more specifically, to an image forming apparatus wherein a toner image formed on a paper sheet is thermally fixed by using a fixing heat roller having disposed therein a plurality of heater lamps.
2. Description of the Related Art
In an image forming apparatus, a fixing heat roller and a pressure roller are utilized to thermally fix a toner image formed on a paper sheet. A plurality of heater lamps such as halogen lamps are disposed within the fixing heat roller for heating the fixing heat roller and, in many cases, each heater lamp is on/off controlled independently with little consideration of the importance of the driving sequence of order of such lamps. If electrical power consumption of the heater lamps is large, electrical variation occurring during on/off controlling is increased and, in some cases, there is a fear that a flicker occurs in a common electrical power system which exceeds a permissible limit.
In the above conventional image forming apparatus, if the electrical power consumption of the heater lamps is large, the electrical variation due to rush current flowing during on/off controlling of the heater lamps is increased, and specifically since the heater lamps are cold when the fixing heat roller is just started and therefore, the resistance thereof is low, there is a fear that a large flicker may occur. To solve such problems, Japanese Patent Application KOKAI Publication No. 2001-142543 proposed that a plurality of heater lamps should be connected in series during the start-up thereof. Since the plural heater lamps are started simultaneously, however, this method cannot sufficiently suppress the flicker.
The present invention has been made in order to solve the above problems, and its object is to provide an image forming apparatus in which a large flicker will not occur when activating heater lamps even if such a fixing heat roller is used as having incorporated therein a heater lamp whose power consumption is large.
To solve this problem, the fixing device according to the present invention comprises a fixing heat roller having disposed therein a plurality of heater lamps and operable to press a paper sheet conveyed between said fixing heat roller and a pressure roller for heat fixing a toner image formed on the paper sheet, and a control operable in activating the heater lamps to drive at least one of said heater lamps earlier than the other heater lamps and to drive said other heater lamps with a predetermined time delay.
By so constructing, in starting the fixing device to heat the fixing heat roller to a predetermined temperature by means of a plurality of heater lamps disposed within the fixing heat roller of the image fixing mechanism, the control can drive, for example, only one heater lamp earlier than the other heater lamps without driving a plurality of or all of the heater lamps, so that the rush current in the driving can be reduced, and since the remaining heater lamps are heated by the earlier driven heater lamp when they are driven with a predetermined time delay, the rush current by such remaining heater lamp can be also reduced. Thus, the fear of a large flicker occurring in the common power system of the image forming apparatus can be eliminated.
In the above fixing device of the invention, said at least one heater lamp of said plurality of heater lamps has a power consumption which is smaller than that of the other heater lamps, and said control is operable in activating the heater lamps to drive firstly said at least one heater lamp whose power consumption is smaller than that of said other heater lamps and to drive said other heater lamps with a predetermined time delay.
By so constructing, the fixing device has the same advantage as that of the image forming apparatus stated above, and since the firstly driven heater lamp has a power consumption smaller than that of the remaining heater lamps, the rush current and hence the flicker in firstly driving the heater lamp can be minimal.
Additionally, in the fixing device according to the present invention, said plurality of heater lamps includes a first heater lamp, and a second heater lamp of which power consumption is larger than that of said first heater lamp, and said control includes a drive command generator for generating first and second pulsed drive commands whose cycles, pulse widths and rise timings are the same and which have first and second voltage levels, respectively, a delay circuit providing said second drive command with a predetermined time delay thereby to generate a third drive command having the second voltage level, a roller temperature detector for detecting the temperature on the surface of said fixing heat roller, a bias supply circuit for supplying first and second bias voltages corresponding to the temperatures detected by said roller temperature detector, a first comparator for comparing the first voltage level of said first drive command and said first bias voltage, a first lamp drive for driving said first heater lamp if said first comparator determines that said first voltage level is higher than said first bias voltage, a second comparator for comparing the second voltage level of said third drive command and said second bias voltage, a second lamp drive for driving said second heater lamp if said second comparator determines that said second voltage level is higher than said second bias voltage.
By so constructing, the control has only to generate the first and second drive commands, and the delay circuit, the first and second comparators and the first and second lamp drives operate in conjunction with each other to cause the first heater lamp whose power consumption is small to be driven firstly and the second heater lamp whose power consumption is larger than that of the first heater lamp to be driven with a predetermined time delay. Because the times at which the first and second heater lamps are driven are thus staggered, a large rush current will not flow at a time. Additionally, since the second heater lamp is heated by the earlier driven first heater lamp when the second heater lamp, whose power consumption is large, is driven, a large rush current due to driving the second heater lamp will not occur. Thus, the fear of a large flicker occurring in the common power system of the image forming apparatus can be eliminated. As mentioned above, because the delay circuit and the first and second comparators are operable to automatically adjust the time of drive command generation in driving of the fixing device and also to automatically control the heater lamp temperature during operation, controlling by the drive command generator can be simplified.
Furthermore, the image forming apparatus according to the present invention comprises a photosensitive body on which a latent image is formed, a developing device for visualizing the latent image on said photosensitive body, a transfer device for transferring the visualized image to a paper sheet, and a fixing device for fixing the transferred image, said fixing device including a pressure roller, a fixing heat roller having disposed therein a plurality of heater lamps and operable in conjunction with said pressure roller to press the paper sheet conveyed thereto for heat fixing a toner image formed on the paper sheet and a control operable in activating the heater lamps to drive at least one of said heater lamps earlier than the other heater lamps and drive said other lamps with a predetermined time delay.
In the above image forming apparatus, said at least one heater lamp of the plurality of heater lamps has a power consumption which is smaller than that of the other heater lamps, and the control is operable in activating the heater lamps to drive firstly said at least one heater lamp and to drive said other lamps with a predetermined time delay.
The present invention further provides a method of heating a fixing heat roller which has disposed therein a plurality of heater lamps and operable to press a paper sheet conveyed between said fixing heat roller and a pressure roller for heat fixing a toner image formed on the paper sheet, comprising the steps of driving at least one of said heater lamps earlier than the other lamps in activating the heater lamps, and driving said other lamps with a predetermined time delay. In this method, the heater lamp which is driven earlier than the other heater lamps has a power consumption which is smaller than that of said other heater lamps.
Additionally, the present invention still further provides a fixing heat roller of a fixing device, which has disposed therein at least two heater lamps and is operable to press a paper sheet conveyed between said fixing heat roller and a pressure roller for heat fixing a toner image formed on the paper sheet, wherein said at least two heater lamps includes, a first heater lamp whose power consumption is smaller than that of the other heater lamps and which is firstly started to be driven in activating the heater lamps, and a second heater lamp whose power consumption is larger than that of said first heater lamp and which is started to be driven with a predetermined time delay after the start of driving said first heater lamp.